1. Field of the Invention
The present invention relates to a semi-insulating InP single crystals used in electronic devices, in particular, OEICs (i.e., optolectronic IC), HEMTs (i.e., high electron mobility transistor) and ion implanted FETs (i.e., field effect transistor), to a process of producing the InP single crystals, to a semiconductor device and a process of producing the semiconductor device, and more particularly to a MISFET (i.e., metal insulator semiconductor FET) and a process of producing the MISFET.
2. Description of the Related Art
For semi-insulating compound semiconductor crystals including Si or S as n-type impurities, a process of adding Fe, Co or Cr as deep acceptors to the crystal has been industrially used. The principle of semi-insulation is based on a mechanism in which deep acceptors compensate shallow donors. Thus, it has been believed that the added amount of an element which act as an acceptor must be larger than the content of donors in the compound semiconductor crystal in order to semi-insulate the crystal.
However, the amount of Fe, Co or Cr doping the compound semiconductor crystals for the semi-insulation is preferred to be as small as possible. This is because Fe, Co and Cr serve as deep acceptors, an ion implanted electronic devices an substrates made of these Fe, Co or Cr doped compound semiconductor crystals, e.g., an FET, and they reduce the activation efficiency of implanted ions in. In the case of devices operating at high frequencies such as an OEIC or HEMT, Fe, Co and Cr diffuse in the epitaxial layers, trap carriers and deteriorate high-frequency and high-speed performances.
In addition, Fe, Co and Cr easily segregate so that the concentrations thereof differ in upper and lower portions of the compound semiconductor crystals, resulting in the nonuniformity of activation efficiency over the crystal, and therefore resulting in low yields of compound semiconductor crystals doped with Fe, Co or Cr.
Heretofore, Fe doped InP single crystals have been generally used for semi-insulating InP single crystal substrates for electronic devices. When the concentration of Fe in InP single crystal is less than 0.2 ppmw, the resistivity is reduced to below 10.sup.6 .OMEGA..multidot.cm and the semi-insulation is deteriorated. Thus, Fe had to be doped with more than 0.2 ppmw in order to retain the semi-insulation thereof.
Generally, it has been believed that a reduced concentration of all of Fe, Co and Cr in the compound semiconductor single crystal reduces the resistivity of the compound semiconductor single crystal since the concentration of a native impurity (i.e., residual impurity) providing a donor amounts to a level of reduced concentration of all of the Fe, Co and Cr.
However, the present inventors proposed that the electrically active point defect, as well as the compensation by donors and deep acceptors characterize the mechanism of semi-insulating the InP single crystal and diligently studied to discover that controlling the density of the point defect by means of heat-treating the InP single crystal caused even a much lower concentration of the deep acceptors than a prior art concentration thereof so as to semi-insulate the InP single crystal or compound semiconductor single crystal.
Thus, the inventors previously provided a process of producing a compound semiconductor having a concentration of 0.2 ppmw or less for all of Fe, Co and Cr and a resistivity of 1.times.10.sup.7 .OMEGA..multidot.cm or more (see Japanese patent application SHO. 63-220632). The technique of the Japanese patent application SHO. 63-220632 is a process which includes the steps of vacuum sealing in a quartz ampoule a compound semiconductor crystal wafer including a concentration of 0.2 ppmw or less of Fe, Co or Cr, placing in the quartz ampoule an element of the compound semiconductor crystal wafer or a compound semiconductor crystal including the element, and heating the quartz ampoule at 400.degree.-640.degree. C. so that the pressure in the quartz ampoule is equal to or higher than a dissociation pressure of the element of the compound semiconductor crystal wafer.
Hofmann et al discloses in "Appl. Phys. A 48, pages 315-319 (1989)" that heat-treating an undoped InP single crystal wafer with a 3.5.times.10.sup.15 cm.sup.-3 concentration of a carrier at a phosphorous vapor pressure of about 5 kg/cm.sup.2 (i.e., 5 bar) at 900.degree. C. for 80 hr, produced an InP wafer having a resistivity of 2.times.10.sup.7 .OMEGA..multidot.cm. This is supposed because the electrically active point defect is concerned in the same manner as in the process of the Japanese patent application SHO. 63-220632.
The present inventors further studied from the process of the Japanese patent application SHO. 63-220632 to discover that even heat-treating an undoped InP single crystal including a concentration of 0.05 ppmw or less for all of Fe, Co and Cr failed to semi-insulate the crystal.
In addition, in the process of Hofmann et al, heat-treating an undoped InP single crystal having a 3.5.times.10.sup.15 cm.sup.-3 carrier concentration deteriorated the mobility from 450 cm.sup.2 /V.multidot.s or more to 300 cm.sup.2 /V.multidot.s or less although the resistivity thereof occasionally was 1.times.10.sup.6 .OMEGA..multidot.cm or more. A significantly high carrier concentration of the undoped InP single crystal provided a resistivity of 10-1.times.10.sup.5 .OMEGA..multidot.cm and seldom achieved a resistivity of 1.times.10.sup.7 .OMEGA..multidot.cm or more. Thus, the present inventors generally reviewed the results of the studies described above and concluded that unless the phosphorous vapor pressure to heat treatment temperature ratio was a limitation, no semi-insulating InP single crystal with a sufficient mobility could be obtained.